Method and apparatus for deriving currents and potentials representative of the impedances of zones of a body

Abstract

In deriving the impedance of a zone of a body errors arise when currents used stray from the zone due to movements of, and/or changes within the body. In the invention a current used for deriving impedance is passed through a zone under investigation using current electrodes outside the zone. A potential also required for deriving impedance is available at two potential sensing electrodes at ends of the zone. The current is confined to the zone by passing control currents just outside the zone between pairs of control current electrodes to set up virtual current barriers minimum along the boundaries of the zone. Pairs of potential sensing electrodes straddling the boundaries are used to adjust currents passed by the control current electrodes to establish the positions of the boundaries and to stabilize these positions when movement and/or internal functional change occurs.

I claim: 
     
       1. Medical investigative apparatus for deriving signals representative of the impedance of a zone of an animal body, comprising means for passing first currents between electrodes of a first group suitable for location on at least one surface of an animal body, the electrodes being, in operation, positioned to pass the first currents through the zone whose irpedance is to be measured,   means for deriving the potential difference across the zone due to the first currents and substantially in the direction of the said first currents,   means for passing second currents through the body between electrodes of a second group also suitable for location on at least one surface of the body to establish virtual barriers generally coinciding with boundaries of the zone within the body, a said virtual barrier being formed along a said boundary when there is no potential gradient in the body perpendicular to the boundary and maximum potential gradient along the boundary,   means for deriving from potentials in the body control signals representative of the positions of the said virtual barriers, and   means for controlling the second currents in accordance with the control signals to control the positions of the said virtual barriers,   the potential difference across the zone being representative of the impedance of the zone.   
     
     
       2. Apparatus according to claim 1 wherein the means for deriving potential difference comprises a plurality of potential sensing electrodes connected to respective high input-impedance amplification means, the potential sensing electrodes being, in operation, positioned in pairs, one on each side of a said virtual barrier, and the means for deriving control signals derives signals representative of the potential differences between electrodes of each pair. 
     
     
       3. Apparatus according to claim 2 wherein each electrode in the first and second groups is connected to a constant alternating-current generator, the generators connected to electrodes in the first group generating a constant amplitude output current, and the generators connected to electrodes in the second group gdnerating output currents with amplitudes which vary according to respective control signals. 
     
     
       4. Apparatus according to claim 2 for use in deriving the impedance of a generally rectangular zone comprising six sensing electrodes positioned on one surface of the body with one sensing electrode at each of opposite ends of the zone between two sensitive electrodes outside the zone but in line with each said end, the first group of electrodes comprising two current electrodes remote from the zone, the second group of electrodes corprising two pairs of current control electrodes, each pair being positioned to pass current outside the zone substantially parallel to the sides thereof and adjacent to two sensing electrodes in line with the opposite ends of the zone. 
     
     
       5. Apparatus according to claim 2 wherein the electrodes are positioned in two arrays each comprising electrodes in the first and second group and sensing electrodes, the arrays being adapted to be positioned on different surfaces of the body, and currents pass through the body from one surface to another. 
     
     
       6. Apparatus according to claim 5 for use in deriving the impedance of a zone which is generally rectangular in cross-section normal to current flow wherein each array comprises a central current electrode which forms a part of the first group of electrodes, eight current control electrodes forming part of the second group of electrodes, the current electrodes forming a rectangular three-by-three array centered on one end of the zone, and four pairs of sensing electrodes, each pair being located in a row or column of the three-by-three array adjacent to the central current electrode, the means for deriving control signals deriving first difference signals from each pair of sensing electrodes to control the current supplied by that one of the said control current electrodes which is external to that pair in the array and in the same row or column, and deriving sum signals from each pair of first difference signals derived from adjacent pairs of sensing electrodes to control the current supplied by current control electrodes at that corner of the three-by-three array which is adjacent to those pairs of sensing electrodes. 
     
     
       7. Apparatus according to claim 5 for use in deriving the impedances of a plurality of zones which are generally rectangular in cross-section normal to current flow, wherein each array is rectangular and comprises a central current electrode which forms part of the first group of electrodes, rows and columns of control current electrodes which form part of the second group of electrodes, and sensing electrodes between the current electrodes in the rows and columns, the seans for deriving control signals deriving a respective control signal to control the current supplied by each control current electrode from sensing electrodes in a region adjacent to, and inward of, that control current electrode in the array, and the current supplied by each current electrode and a potential derived from at least one of the sensing electrodes adjacent to that current electrode being the current and potential representative of the impedance of a zone centred on that current electrode. 
     
     
       8. Apparatus according to claim 7 wherein the means for deriving control signals derives control signals representative of the value: ##EQU5## where P n  is the potential at the n th  sensing electrode from the center of a row or column and S n  is the distance through the body from the n th  electrode in one array to the corresponding electrode in the other array. 
     
     
       9. Apparatus according to claim 1 wherein the first and second groups of electrodes include an electrode common to both groups and except for the cormon electrode, each electrode in the first and second groups is connected to a constant alternating-current generator, the generators connected to electrodes in the first group generating a constant amplitude output current, and the generators connected to electrodes in the second group generating output currents with amplitudes which vary according to respective control signals. 
     
     
       10. Apparatus according to claim 9 for deriving current and potential signals representative of the impedances of zones of the human body containing significant portions of the lungs, comprising two common electrodes one located over the sternum and one located over the spine, a plurality of current electrodes spaced apart along the sides of the body in the region of the chest, with sensing electrodes between the current electrodes, one current electrode on each side of the body being in the first group of electrodes, the other current electrodes being in the second group, and the current in each electrode in the second group being controlled by the potential difference between sensing electrodes adjacent thereto but on that side thereof adjacent to the nearest electrode in the first group. 
     
     
       11. Apparatus for deriving signals representative of the impedance of a zone of a cross-section of a closed body, comprising an assembly including a supporting member, a plurality of electrode holders mounted thereon, the supporting member being arranged to encircle, in operation, a portion of the human body, and electrodes held by the said holders, the said holders being adapted to hold the said electrodes in contact with the surface of the said body,   means for passing first currents between a plurality of the electrodes in a first group positioned to pass the first currents through the zone whose impedance is to be measured,   means for deriving the potential difference across the zone due to the first currents and substantially in the direction of the said first currents,   means for passing currents through the body between a plurality of the electrodes in a second group positioned in relation to electrodes of the first group to establish virtual barriers generally coinciding with boundaries of the zone within the body, a said virtual barrier being formed along a said boundary when there is no potential gradient in the body perpendicular to the boundary and maximum potential gradient along the boundary,   
     
     
       means for deriving from potentials in the body control signals representative of the positions of the potential barriers, and means for controlling the second currents in accordance with the control signals to control the positions of the potential barriers, the potential difference across the zone being representative of the impedance of the zone.   
     
     
       12. Apparatus according to claim 11 wherein the means for deriving potential difference comprises a plurality of potential sensing electrodes connected to respective high input-impedance amplification means, the potential sensing electrodes being, in operation, positioned in pairs, one on each side of a said virtual barrier, and the means for deriving control signals derives signals representative of the potential differences between electrodes of each pair. 
     
     
       13. Apparatus according to claim 12 wherein each electrode in the first and second groups is connected to a constant alternating-current generator, generators connected to electrodes in the first group generating a constant amplitude output current, and generators connected to electrodes in the second group generating output currents with amplitudes which vary according to respective control signals. 
     
     
       14. Apparatus according to Claim 2 comprising means for applying forces to the holders automatically to cause the holders to press on to the body with equal contact pressure. 
     
     
       15. Apparatus according to claim 11 including means for generating respective signals representative of the displacements of each electrode from a circular datum. 
     
     
       16. Apparatus according to claim 11 including means for generating respective signals representative of the inclinations of the holders from the perpendicular to the plane of encirclement of the electrode holders. 
     
     
       17. Apparatus according to claim 11 wherein the said zone passes through the center of a cross-section of the body, the apparatus including means for supplying one electrode in each holder with a current according to the expression: ##EQU6## where I o  is the current passed between electrodes at the ends of said zone, n is the number of electrode holders with n=0 for the electrode holders of the central zone,   r n  =R-d n     R is the radius of the ring formed when all the electrode holders have zero displacement towards the center of the body,   d n  is the displacement of the n th  holder towards the center of the body,   r.sub.(-1) is the value of r n  for zone on the opposite side of the central zone from the zone with the value r 1 , and   θ n  is the angle between the radii passing through the electrodes corresponding to n=0 and n.   
     
     
       18. Apparatus according to claim 11, wherein the means for deriving control signals derives control signals representative of the value: ##EQU7## where n is the number of electrode holders with n=0 for the electrode holders of the central zone, R is the radius of the ring formed when all the electrode holders have zero displacement towards the center of the body,   P n  and P n+1  are the potentials of sensing electrodes in the n th  and (n+1) th  holders, respectively, and   θ n  is the angle between the radii passing through the electrodes corresponding to n=0 and n.   
     
     
       19. A method of deriving the impedance of a zone of a body using electrodes placed in contact with the body wherein the dimensions of the zone are comparable with the maximum distances between the electrodes, comprising positioning and energizing electrodes of a first group on the surface of the body to pass first currents through the zone,   deriving the potential difference across the zone due to the first currents and substantially in the direction of the said first currents,   positioning and energizing electrodes of a second group on the surface of the body to establish virtual barriers generally coinciding with boundaries of the zone within the body by passing control currents between the electrodes of the second group, a said virtual barrier being formed along a said boundary when there is no potential gradient in the body perpendicular to the boundary and maximum potential gradient along the boundary,   adjusting the control currents in accordance with potentials representative of the positions of the virtual barriers to control the said positions, and   deriving an indication of the impedance of the zone from the said potential difference across the zone.